Imaging Cerenkov emission as a quality assurance tool in electron radiotherapy

A new potential quality assurance (QA) method is explored (including assessment of depth dose, dose linearity, dose rate linearity and beam profile) for clinical electron beams based on imaging Cerenkov light. The potential of using a standard commercial camera to image Cerenkov light generated from electrons in water for fast QA measurement of a clinical electron beam was explored and compared to ionization chamber measurements. The new method was found to be linear with dose and independent of dose rate (to within 3%). The uncorrected practical range measured in Cerenkov images was found to overestimate the actual value by 3 mm in the worst case. The field size measurements underestimated the dose at the edges by 5% without applying any correction factor. Still, the measured field size could be used to monitor relative changes in the beam profile. Finally, the beam-direction profile measurements were independent of the field size within 2%. A simulation was also performed of the deposited energy and of Cerenkov production in water using GEANT4. Monte Carlo simulation was used to predict the measured light distribution around the water phantom, to reproduce Cerenkov images and to find the relation between deposited energy and Cerenkov production. The camera was modelled as a pinhole camera in GEANT4, to attempt to reproduce Cerenkov images. Simulations of the deposited energy and the Cerenkov light production agreed with each other for a pencil beam of electrons, while for a realistic field size, Cerenkov production in the build-up region overestimated the dose by +8%.